User:Kary Atkinson

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Introduction
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Glutamine synthetase is composed of twelve subunits, formed as two hexameric rings. The two rings are held together by twelve central loops TextToBeDisplayed which extend into the central region of the complex and include a small, four stranded β loop TextToBeDisplayed. Each loop is 33 residues long and has a convex shape. The spatial orientation TextToBeDisplayed of the loops has been found to be less accessible for interactions to occur, contributing to the catalytic activity of the enzyme.

The central loop (residues 156-188) is a site where covalent modifications, with inhibitory effects are found. This segment of the complex is subject to proteolysis by four secreted proteases, which cleave specific residues. Another covalent modification known to occur at central loop of Glutamine synthetase is ADP-ribosylation of Arg-172 TextToBeDisplayed.

It is evident then that the central loop of glutamine synthetase (residues 156-188) is an important component of the enzyme, anchoring the subunits together and providing the spatial orientation necessary for activity. Therefore the central loop contributes both to the function and stabilization, via numerous hydrophobic interactions TextToBeDisplayed and four different hydrogen bonding interactions TextToBeDisplayed of the quartenary structure of Glutamine synthetase.

References:
1.  Fisher, M. T., Stadtman, E. R., Oxidative Modification of Escherichia coli Glutamine Synthetase, The Journal of Biological Chemistry, 1992, Vol 267. No3, 1872-1880

2.  Moss, J., Stanley, S., Levine, R. L., Inactivation of Bacterial Glutamine Synthetase by ADP-ribosylation, The Journal of Biological Chemistry, 1990, Vol. 265, No. 34, 21056-21060

3.  Yamashita, M. M., Almassy, R. J., Janson, C. A., Cascio, D., Eisenberg, D., Refined Atomic Model of Glutamine Synthetase at 3.5 A Resolution, The Journal of Biochemistry, 1989, Vol. 264, No. 30, 17681-17690